Update atom-shell-vs-node-webkit.md

[makerGeek]

nwjs now supports a js script as entrypoint.
cf. http://docs.nwjs.io/en/latest/References/Manifest%20Format/#main

[welcome]

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• Follow the JavaScript, C++, and Python coding style.
• Run npm run lint locally to catch formatting errors earlier.
• Document any user-facing changes you've made following the documentation styleguide.
• Include tests when adding/changing behavior.
• Include screenshots and animated GIFs whenever possible.
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[welcome]

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